Add Other Means to Add Contacts

i like the p2p and secure concept of briar.
even if simpler on android, resource consumption is much more controlled than tox and so usable.

local contact addition by bluetooth only is a bit too strict.

i see the discussion about QR codes with direct exchange #117 (closed)
but it seems it is also a direct contact

contact recommendation can be nice

but i would like a standard contact invitation by email, phone call, or sms.
i know this is less secure but for people we do not see regularly this is better than keeping with normal app.
maybe sharing an invitation key with any kind of prefered app is the easier thing